Utterly Mad

IDK where this should go exactly but I've got a question, well maybe two IDK.
Do you guys have any suggestions on how to write a setting for something like DND or other such things?
I keep trying but I either end up feeling like it's TOO detailed for my liking or it's too vague to be playable, so any suggestions for how to write a setting?

My advice for settings is to set only the vitally important to the campaign stuff in stone, and make the rest up as you go.